1. Field of the Invention
The invention disclosed in this specification and the like relates to a semiconductor device and a manufacturing method thereof.
In this specification and the like, a semiconductor device refers to all types of devices which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of semiconductor electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Further, Patent Document 3 discloses a transistor structure as follows: the transistor includes a source electrode and a drain electrode in contact with an oxide semiconductor layer, a gate electrode overlapping with the oxide semiconductor layer, and a gate insulating layer between the oxide semiconductor layer and the gate electrode; and the source electrode and the drain electrode includes a first conductive layer and a second conductive layer having a region which extends in a channel length direction beyond an end portion of the first conductive layer.